Various research and development have hitherto been conducted on a sintering process, sintering aids, a limitation on the crystal phase to be involved, etc., for the purpose of improving the strength of silicon nitride material. For example, regarding the sintering process, a bending strength of about 100 kg/mm.sup.2 has been realized in a hot press sintering process as described in Am. Ceram. Soc. Bull., 52 (1973) pp. 560. Further, a hot isostatic pressing process (HIP process) wherein use is made of a glass capsule has also been developed.
Although these techniques can provide excellent properties in respect of the strength of the sintered body, they are not advantageous in respect of the productivity and cost efficiency. In order to solve this problem, a gas pressure sintering process see, for example, Mitomo, "Funtai to Kogyo" Vol 12 No. 12, pp. 27 (1989)! has been proposed in the art. In this process, since the densification of a final sintered body is accompanied by the grain growth of a .beta.-silicon nitride crystal, there is a high possibility that the strength might be deteriorated due to the precipitation of coarse crystal grains. Further, since sintering is conducted under a nitrogen gas pressure of 10 atm or more, as with the hot press process and HIP process, the size of necessary sintering equipment becomes large. Therefore, this process has not been regarded as a satisfactory technique in respect of the properties and productivity. On the other hand, regarding a sintering aid, an Si.sub.3 N.sub.4 -Al.sub.2 O.sub.3 -Y.sub.2 O.sub.3 -based silicon nitride sintered body, wherein Y.sub.2 O.sub.3 is used as a main aid, is disclosed in Japanese Patent Publication Nos. 21091/1974 and 38488/1973. As described in the specifications of these publications, it is thought that the crystal grain of .beta.-silicon nitride forms a fibrous structure in the sintered body, which is dispersed in the matrix, thus contributing to an improvement in the strength and toughness. Specifically, this technique positively utilizes a phenomenon that since the crystal form of B-silicon nitride is hexagonal, the crystal anisotropically grows in the direction of C-axis. In particular, as described in the Japanese Patent Publication No. 38448/1973 and Journal of Ceramic Society of Japan, Vol. 94, pp. 96 (1986), in some cases, fibrous .beta.-silicon nitride crystal grain grows into a size of ten-odd .mu.m or more in the direction of C-axis. In this technique, however, there is a possibility that the growth of the grain might lead to abnormal growth or occurrence of pores, which in turn gives rise to a deterioration in the strength. Further, in this process wherein use is made of a sintering aid, no satisfactory densification can be attained without raising the sintering temperature to 1700.degree. to 1900.degree. C., and when sintering is conducted under a nitrogen gas pressure around atmospheric pressure, sublimation and decomposition of silicon nitride occur, so that no stable sintered body can be produced in some cases. For this reason, the above process has not been regarded excellent in respect of both the properties of the sintered body and the productivity. In all the above-described techniques, the strength of the resultant sintered body is about 100 kg/mm.sup.2 at the highest in terms of the three-point bending strength according to JIS R 1601, and the properties are not always satisfactory when various applications of silicon nitride materials are taken into consideration.
Examples of the application of the ceramic material to automobile parts include applications to an impeller of a turbo charger (see, Shibata, Hattori and Kawamura, "New Ceramics Journal", No. 1, pp. 91 (1988)! and a tappet shim for a diesel engine see Hara, Kobayashi, Matsui and Akabane, "Jidosha Gijutsu" Vol 45 No 4 pp 33 (1991)!. All of these examples utilize the properties of the silicon nitride ceramics, such as weight reduction and abrasion resistance. When the silicon nitride ceramics are utilized as valve train materials or mechanical parts, there occurs a problem of a reliability under the present conditions. In particular, the development of a silicon nitride material excellent in the abrasion resistance as well as in the impact resistance has been desired in the art because when parts such as a tappet shim are used in a usual reciprocating engine, a very high impact stress occurs in an unstationary operating condition (for example, surging phenomenon).
An object of the present invention is to provide a technique capable of satisfying both requirements for productivity and mechanical properties of the sintered body unattainable in the above-described prior art.